But should he prove successful in representing the couple while they remain official suspects in the case, he could have lucrative prospects in the PR industry.
The 46-year-old former BBC reporter and presenter was seconded by the Foreign Office in May to help the couple cope with the extraordinary media presence in the resort of Praia da Luz.
He spent almost a month with them, often working 14-hour days as he helped to set up their visits to different cities and briefed journalists.
Unlike his predecessor Justine McGuinness, he will not be paid from the £1million-plus Find Madeleine fund.
His unspecified salary will be met by "an anonymous financial backer".
Sir Richard Branson has donated £100,000 to the McCanns' legal costs. It was not known if his money was paying for Mr Mitchell.
Once the Madeleine case is over Mr Mitchell will continue to work for the McCanns' anonymous backer and will not return to Whitehall.